Sorbic acid and its salts (e.g. potassium salt) are antifungal and effective in preserving food so that they are in broad use as food additives.
The representative commercial technology for the production of sorbic acid or a salt thereof comprises reacting crotonaldehyde with ketene to give a polyester, decomposing the polyester with an acid or alkali to give sorbic acid or a salt thereof in crude form, and purifying the crude sorbic acid or salt by crystallization. This crystallization is generally effected by dissolving said crude sorbic acid or salt in hot water and cooling the solution to cause sorbic acid or its salt to separate out or by dissolving said crude sorbic acid in an aqueous alkaline solution to give an alkali metal sorbate solution and adjusting the solution to pH about 2-3 with an acid such as hydrochloric acid to thereby cause sorbic acid crystals to separate out.
In the above production process, large amounts of effluents containing sorbic acid or its salt are formed in the step of producing crude sorbic acid from said polyester and the subsequent crystallization step. For example, in the step where the polyester is decomposed with hydrochloric acid to give crude sorbic acid, the filtrate after recovery of sorbic acid contains not only organic matter inclusive of sorbic acid and reaction byproducts but also tarry matter so that its BOD (biological oxygen demand) is of the order of tens of thousands to hundreds of thousands parts per million (ppm). Moreover, said organic matter may comprise as many as 50 or more different substances. The filtrate in said crystallization step also contains sorbic acid or its salt and substantially the same amount of other organic matter (about 30-40 kinds of substances) and its BOD is as high as 2000-10000 ppm. Incidentally, the solubility of sorbic acid in water is about 0.1-0.3 weight % at room temperature, although it is increased to about 3 weight % at elevated temperature.
Generally, these effluents are subjected to an activated sludge or other biological treatment and then allowed to drain into a public waterway. However, the effluent from the above sorbic acid production and purification line has such a high BOD value that the load on the biological treatment system is large and therefore, its disposal requires an enormous equipment and running expenditure.
As alternative technologies for the production of sorbic acid, there are known the process which comprises decomposing .gamma.-vinyl-.gamma.-butyrolactone and the process which comprises decomposing acetoxyhexenoic acid which is the reaction product of butadiene with acetic acid. However, in these processes, too, large volumes of effluents containing sorbic acid or its salt are discharged in the production and purification stages, thus presenting the same problems as mentioned above.
Meanwhile, there has been reported a method which comprises acidifying a dilute aqueous solution of sorbic acid or its salt, concentrating it to about 5-fold the initial concentration by the reverse osmosis technique, and recoverying sorbic acid or its salt from said aqueous solution. However, the membrane effluent shows a high BOD value and cannot be directly allowed to drain into a public waterway.